Cathode



Feb. 21, 1956 F. P. GAGL!ANO RECOVERY OF GOLD FROM SCRAP CONTAINING ITCATHODE Filed Feb. 10, 1954 ANODE CONDUCTOR CATHODE "'1 GOLD SCRAPELECTROLYTE CONTAlNING FERROCYANDE- INVENTOR fiankfl Gaylz'ano UnitedStates Patent RECOVERY OF GOLD FROM SCRAP CONTAINING 1T Francis P.Gagliano, Jackson Heights, N. Y., assignor to Secon Metals Corporation,White Plains, N. Y., a corporation of New York Application February 10,1954, Serial No. 409,475

8 Claims. (21. 204-410 This invention relates to the recovery of goldfrom gold plated or clad material, such, for example, as the scrapproduced in the manufacture of grids and other parts of electronic tubesand other sources of gold plated or clad material.

Substantial amounts of scrap in the form of gold plated or clad wire,ribbon, sheets, etc., which scrap contains gold in amount of from 0.1%to 6%, copper or silver or both in amounts of from less than 1% to ashigh as 90% of copper and from less than 1% to as high as 10% of silver,is produced in the manufacture of electronic tubes. Such scrap maycontain other metals including one or more of the followingnickel, iron,tungsten, molybdenum and cadmium.

All percentages and parts herein are on a weight basis.

Attempts heretofore made to recover the gold content of such scrap byelectrolysis employing a cyanide as the electrolyte have usuallyresulted in the copper or silver plating out preferentially to the goldand otherwiseinterfering with the recovery of the gold to an extentrendering such process uneconomical and inefiicient, particularly inthat the recovery process is very slow. It has been found that, as theconcentration of the copper or silver builds up in the electrolyticbath, the eificiency of the bath decreases until it reaches a pointwhere it can no longer be used practically to efiect the recovery of thegold.

This invention is predicated on the surprising and unexpected discoverythat the gold content of gold plated or clad material containing copperor silver or both can readily and efiiciently be recoveredelectrolytically by employing a bath containingfrom about 3% to about16% alkali metal or ammonium ferrocyanide, from about 0.25% to about 8%alkali metal or ammonium cyanide, and maintaining the bath at atemperature of from 20 to 80 C., and at a pH of from about 7.5 to about12. The ratio of the ferrocyanide to the cyanide in the bath should bewithin the range of from 2:1 to 60:1.

Preferably, the bath contains from about 2% to 5% alkali metal orammonium ferrocyanide, from 0.5% to 2% alkali metal or ammonium cyanide,a ratio of ferrocyanide to cyanide of 521. The bath, in use, ismaintained at a temperature offrom about 40 to 50 C. and a pH of from 8to 10. The ferrocyanide, cyanide and salts or alkali hydroxides addedfor purposes of maintaining the desired pH, preferably have the 'samecation. Thus, when employing potassium ferrocyanide as a constituent ofthe electrolytic bath, potassium cyanide is used along with potassiumcarbonate, potassium phosphate and/ or potassium hydroxide, as will beexplained more fully hereinafter. Y

Particularly preferred is the use of a bath containing 4% potassiumferrocyanide, 0.8% potassium cyanide, 0.5 potassium carbonate at atemperature of about 45 C. and a pH of about 9. This pH is maintained bythe addition of monopotassium phosphate (KH2PO4) and potassium hydroxideas needed.

As the ferrocyanideemployed in the bath, potassium or sodiumferrocyanide is preferred, although other alkali 2,735,810 Patented Feb.21, 1956 or francium ferrocyanide may be used. Ammonium ferrocyanide mayalso be employed.

As the cyanide, sodium or potassium cyanide is preferred, although theother above-mentioned alkali metal or ammonium cyanides may be used. Thecyanide is added to'the bath because it forms complexes with metals,such as iron, copper, nickel, silver, chromium, etc., which complexesare soluble and remain in solution. In the absence of the cyanide,insoluble compounds, such as Fe(CN) 2, a brown precipitate, Fe2[Fe(CN)1s, Fe(OH)s, Fe(OH)z, Ni(CN)2, CuzFe(CN)s and Cu4Fe(CN)s result whichcontaminate the bath and reduce its efficiency. As the electrolysisproceeds employing a bath having the composition above noted the free CNconcentration decreases due to the formation of complexes of the typeabove noted. Accordingly, from time to time, as needed, additionalcyanide is added to replenish the cyanide thus used up and maintain itscencentration within the range above noted.

As above indicated, preferably, the cation of the ferrocyanide and thecyanide, as well as of any other salts or bases incorporated in thebath, are the same.

In order to maintain the pH within the range above noted, alkali metalor ammonium carbonates, bicarbonates, hydroxides or phosphates, ormixtures of these salts 'or bases may be added to the bath as neded.Desidably, a mixture of alkali metal phosphate and carbonate is used.The alkali metal carbonate serves to maintain the pH above 7.5, and thephosphate appears to have a buffer action to maintain the pH at thedesired value substantially constant over relatively long periods oftime.

A bath having the composition above noted has the advantage ofminimizing the efiluent disposal problem. Spent baths employed in thisinvention can readily be treated, for example, by chlorination, tocondition them to provide an eflluent which can be disposed of in theusual sewage or other industrial drain systems.

Moreover, a ferrocyanide-cyanide bath containing at least 2 parts offerrocyanide per part of cyanide is much less toxic than the cyanidebaths heretofore employed. This is the case, because the ferrocyanidesare much more stable than the cyanides. Furthermore, in view of therelatively small amounts of cyanide contained in the baths employed inpracticing this invention, there is ma terially less tendency forevolution of hydrogen cyanide to take place'than there is from thecyanide baths heretofore used.

The cathode may be any desired electrical conductor, preferablymetallic, on which the gold will deposit. Thus, gold plates or bars maybe used as the cathode.

, Alternatively, stainless steel may be used from which the metalferrocyanides, suchas lithium, rubidium, caesium,

with contact rods on which the charge of scrap metal is supported sothat the electrical current is supplied efficiently to the scrap metal.-It will be understood the anode simply serves as an electricalconductor for supplying current to the scrap metal and that in effectthe scrap metal is the anode, once electrolysis has commenced.

The voltage and current density used will, of course, depend on theparticular equipment employed, the size of the bath, the size of thecathodes, anodes, etc. In the recovery of gold from scrap employing arelatively small bath for treating charges of from about to about 500grams, from 2 to 6 volts and a current density of from 1 to 6 amperesper square foot have been used with satisfactory results. However, asabove noted, the voltage and current density may be varied within widelimits and will depend on the equipment used.

Metal which may be treated in accordance with this invention containsfrom 0.1% to 6% gold, an appreciable amount of either silver or copperor both, the copper content not exceeding 90% and the silver content notexceeding 10%, from to 90% nickel, from 0% to 80% iron, from 0% to 94%tungsten, from 0% to 94% molybdenum, and from 0% to 20% chromium. Itwill be understood the process of this invention is applicable not onlyto the recovery of gold from such scrap, but also from any gold clad orplated material containing other metals, particularly copper or silveror both in amounts of from 1% to 90% copper, if copper is present, andfrom 1% to silver, if silver is present.

The charge is left in the bath as long as it is necessary to remove orstrip the gold. This usually will be evident from a change in color ofthe charge. The charge should not be left in the bath for an excess oftime beyond that necessary to strip substantially all of the gold,because this will result in unnecessary contamination of the bath.

In the accompanying drawing is shown a vertical section through one formof an electrolytic bath which may be employed in practicing thisinvention. It will be understood this showing is for purposes ofillustration only and other types of equipment may be used.

In the single figure of the drawing, 10 indicates a container providedwith an acid resistant lining 11, desirably of polyethylene, rubber, orother plastic. Cathodes 12, which in the embodiment shown, are ofstainless steel, are immersed in the bath 13. Centrally disposed withinthe container 10 is a perforated basket 14, desirably of acid resistantplastic material, such as polyethylene. Basket 14 receives the charge ofscrap 15 to be treated. Anode conductor 16 of any electricallyconducting material, e. g., copper, steel, etc., is provided withcontact rods 17 which extend through the pile of scrap 15 providingsatisfactory electrical contact therewith.

The following examples are given for purposes of illustrating theinvention; it will be understood the invention is not limited to theseexamples. In all of these examples the material treated was industrialscrap produced in the production of grids for electronic tubes. Itcontained approximately 0.75% gold, 10% copper, 70% iron and nickel andthe rest molybdenum and tungsten. All examples were carried out inequipment of the type shown in the drawing employing a stainless steelcathode. Upon the stripping of the gold from each charge or during eachrun, the charge was removed and a fresh charge introduced.

EXAMPLE I The bath used in this example was made by mixing 600 grams ofpotassium ferrocyanide K4Fe(CN)s3H2O, 10 grams of potassium cyanide and1 gallon of water. The resultant solution was used as the electrolyticbath in recovering the gold content of 6 successive charges of thescrap. The data on these 6 charges (referred to as Run 1, 2, etc. in thetable) is given in Table 1 which follows:

Analysis of the cathode plate showed it contained 40% gold. The goldremoved from the cathode could readily be recovered in substantiallypure form, for example, by subjecting it to an acid refining to removethe other metal chiefly copper.

It is noted that in a comparative run using the same scrap but in whichthe bath consisted of a solution of potassium cyanide, the bath was soonfouled and it was impossible to recover the gold content of even onecharge of about 200 grams.

EXAMPLE II The composition of the bath was 500 grams of potassiumferrocyanide, grams potassium cyanide to 1 gallon of water. Potassiumhydroxide was added after the second run to bring the pH of the bath upto 12. Additional potassium hydroxide was added at the beginning of Run4 to maintain the pH of the bath at 12. Data on these four runs isgivenin Table 2 which follows:

Table 2 l'ilaime in inutes iit Taken to Tom era- Run char 6 RecoverVoltage Amperage pH of N0. Substan- Used Used Bath B g tially a I GramsA of the Gold The cathode plate was heavy and showed a 60% gold contentcorresponding approximately to an 80% gold recovery from the goldcontent of the scrap.

EXAMPLE III The composition of the bath used in this example was 200grams potassium ferrocyanide, 75 grams potassium cyanide, 40 gramspotassium carbonate and 1 gallon of water. At the conclusion of Run 3 asmall amount of monopotassium phosphate was added, also small amounts ofpotassium hydroxides were added to the bath to maintain the pH at thevalues indicated. Data on these runs are given in Table 3 which follows:

Table 3 Time in Weight ,ilgg fif Run ef 8 Recover Voltage Amperage pH ofTEE???" No. st gs en- Used Used Bath Bath 00 y Grams An of the GoldAnalysis of the cathode plate showed approximately 100% gold recoveryfrom the gold content of the scrap.

The expression gold plated meta is used in the claims in a broad senseand is intended to include gold clad materials as well as plated.

Since certain changes may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:

1. An electrolytic process of recovering gold from gold plated metals,which comprisessubjecting the gold plated metal to electrolysisemploying an electrolytic bath containing a ferrocyanide from the groupconsisting of alkali metal and ammonium ferrocyanide and a cyanide fromthe group consisting of alkali metal and ammonium cyanides, the ratio offerrocyanide to cyanide in the bath being within the range of from 2:1to 5:1 by weight.

2. The process as defined in claim 1, in which the bath contains from 3%to 16% by weight of ferrocyanide and from 0.25 to 8% by weight ofcyanide.

3. A process of recovering gold from gold plated metals, which comprisessubjecting the gold plated metals to electrolysis employing anelectrolytic bath containing from 3% to 16% by weight of a ferrocyanidefrom the group consisting of the alkali metal and ammoniumferrocyanides, from 0.25 to 8% by weight of a cyanide from the groupconsisting of by weight, the alkali metal and ammonium cyanides, theratio of ferrocyanide to cyanide being within the range of from 2:1 to5:1, the bath during the electrolytic decomposition being maintained ata pH of from 7.5 to 12 and at a temperature of from to 80 C.

4. A process of recovering gold from gold plated metals, which comprisessubjecting the gold plated metals to electrolysis employing anelectrolytic bath containing from 2% to 5% by Weight of a ferrocyanidefrom the group consisting of the alkali metal and ammoniumferrocyanides, from 0.5% to 2% by weight of a cyanide from the groupconsisting of the alkali metal and ammonium cyanides, the ratio offerrocyanide to cyanide being about 5:1 by weight, the bath during theelectrolytic decomposition being maintained at a pH of from 8 to 10 andat a temperature of from to C.

5. A process of recovering gold from gold plated metals, which comprisessubjecting the gold plated metals to electrolysis employing anelectrolytic bath containing about 4% by weight of an alkali metalferrocyanide and about 0.8% by weight of an alkali metal cyanide, thebath during the electrolytic decomposition being maintained at a pH ofabout 9 and at a temperature of about 45 C.

6. A process of recovering gold from gold scrap containing approximatelyfrom 0.1% to 6% by weight of gold and containing a metal from the groupconsisting of copper and silver, which comprises subjecting the scrap toelectrolysis in an aqueous bath containing from 3% to 16% by weight offerrocyanide from the group consisting of alkali metal and ammoniumferrocyanides, and from 0.25% to 8% by Weight of a cyanide from thegroup consisting of alkali metal and ammonium cyanides at a pH of from7.5 to 12 and a temperature of from 20 to C., the ratio of ferrocyanideto cyanide being within the range of from 2:1 to 5:1 by Weight.

7. The process as defined in claim 6, in which the bath contains from 2%to 4% by Weight of potassium ferrocyanide, from 0.5% to 2% by Weight ofpotassium cyanide, potassium carbonate and potassium phosphate in amountto maintain the bath at a pH of about 9 and the temperature of the bathis maintained at approximately 45 C.

8. An electrolytic process of recovering gold from gold-containingmetals, which comprises introducing the gold-containing metals into anelectrolytic bath containing a ferrocyanide from the group consisting ofalkali metal and ammonium ferrocyanides and a cyanide from the groupconsisting of alkali metal and ammonium cyanides, the gold-containingmetal thus introduced being made the anode, said bath also containing acathode, the ratio of ferrocyanide to cyanide in the bath being withinthe range of from 2:1 to 5:1 by weight, and passing current through saidbath to eiiect deposition of the gold on said cathode.

References Cited in the file of this patent Kern: Transactions of theElectrochemical Society, vol. 24 (1913), pp. 241 to 270. Pages 260 to265 and 268 pertinent.

Kushner: Products Finishing, January 1942, pages 46 to 56.

Kushner: Products Finishing, December 1941, pages 26 and 28.

1. AN ELECTROLYTIC PROCESS OF RECOVERING GOLD FROM GOLD PLATED METALS,WHICH COMPRISES SUBJECTING THE GOLD PLATED METAL TO ELECTROLYSISEMPLOYING AN ELECTROLYTIC BATH CONTAINING A FERROCYANIDE FROM THE GROUPCONSISTING OF ALKALI METAL AND AMMONIUM FERROCYANIDE AND A CYANIDE FROMTHE GROUP CONSISTING OF ALKALI METAL AND AMMONIUM